as a special case of the preceding discussion the investor may wish to divide their portfolio between a risk-free investment such as a savings account and a risky investment such as a security. The risk-free interest rate will be symbolized by r. By assumption the rate of return on the risk-free investment is guaranteed, i.e. has a variance of zero. If the rate of return of the security is r and x is invested in the security while 1-x is placed in the risk-free investment, the rate of return on the portfolio is XXX